A
cell wall is a more or less solid layer surrounding a
cell[?]. They are found in most
bacteria,
fungi,
plants, and
algae.
Animals and most other
protists have
cell membranes without surrounding cell walls.
Plant cell walls have two main functions: to provide rigidity for the cell, and to prevent expansion when water enters the cell. Made of insoluble
cellulose fibres meshed in to a matrix of
carbohydrates called
pectates and
hemicelluloses, they give the plant strength and support. The cell wall is generally permeable to anything entering the cell in solution unless impregnated with
lignin in wood or
suberin[?] in cork
tissue to produce
wood.
The primary cell wall, built by the plant first, is composed of cellulose microfibrils[?] aligned at all angles. Microfibrils are held together by hydrogen bonds to provide a high tensile strength. After the maximum size necessary has been reached, a secondary wall is constructed below. Unlike the primary wall, the microfibrils are aligned mostly in the same direction, and with each additional layer the orientation changes slightly.
Cell walls of neighbouring cells are held together by a shared gelatinous membrane called the middle lamella, which contains magnesium and calcium pectates[?] (salts of pectic acid[?]).
Cell walls of bacteria are primarily used for protection against hostile envorinments or, in the case of
pathogenic bacteria, against the
immune system of the
host. They contain
peptidoglycan, which can be made visible in
Gram-positive bacteria by
Gram staining. The cell walls of bacteria are also vital for containing the high
osmotic pressure inside bacterial cells caused by the high concentration of
solutes in the
cytoplasm, which can often be as high as 15 atmospheres. Many
antibiotics, including
penicillin and its derivatives, target the cell wall of bacteria.
The cell walls of fungal cells are composed of chitin, the same carbohydrate that gives strength to the exoskeletons of insects. They serve a similar purpose to those of plant cells, giving fungal cells rigidity and strength to hold their shapes.
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